Violent offenses are rising in midsize cities for the first time in a decade. A look at what's behind the surge  and why it won't be slowing anytime soon

It's as if Milwaukee, Wis., had reverted to a state of lethal chaos. A Special Olympian is killed for his wallet as he waits for a bus. An 11-year-old girl is gang-raped by as many as 19 men. A woman is strangled, her body found burning in a city-owned garbage cart. Twenty-eight people are shot, four fatally, over a holiday weekend.

These are the kinds of crimes American cities expected never to see in high numbers again. In the 1990s police departments nationwide began applying the so-called broken-windows theory: arrest the bad guys for minor offenses, and they wouldn't be around to...